(1) Field of the Invention
This invention relates to the manufacture of integrated circuit devices and more particularly to application and processing of spin-on-glasses.
(2) Description of prior art
The manufacture of large scale integrated circuits in a mass production facility involves hundreds of discrete processing steps beginning with the introduction of blank semiconductor wafers at one end and recovering the completed chips at the other. The manufacturing process is usually conceived as consisting of two segments The first is often referred to as front-end-of-line(FEOL) processing and includes those processing steps wherein the semiconductor devices are formed within the silicon surface. The second segment called back-end-of-line(BEOL) processing encompasses the formation of the various layers of interconnection metallurgy above the silicon surface. Depending on the complexity of the product and the circuit design as many as four layers of metallurgy may be required to provide the required circuit interconnections and connecting pads for the chips external connections.
The processing steps in BEOL involve depositing layers of material, patterning them by photolithographic techniques, and etching away the unwanted portions. The materials consist primarily of insulators and metal alloys.
Each time a layer of metallization is applied and circuit paths are etched within it, the next layer of insulation is preferred to flow into the contours of the metal pattern it is covering so as to render a smooth planar surface for the next metal layer. Most traditional film deposition techniques such as low-pressure-chemical-vapor-deposition(LPCVD), plasma-enhanced-chemical-vapor-deposition(PECVD), and rf sputtering cannot provide this desired surface planarization. These processes are more or less conformal and thereby replicate the topography upon which they are deposited. Only materials which can experience liquid flow during deposition are capable of providing such a planar upper surface. To this end low melting glasses such as phosphosilicate glass(PSG) can be deposited by LPCVD or PECVD and then caused to flow at elevated temperatures. However, because of the temperatures required for adequate flow, they have only found use prior to the deposition of the first layer of metallization.
Other alternatives comprise materials that can be applied in liquid form and subsequently hardened or cured to form solid insulating layers. In this category are photoresists, polyimides, and spin-on-glasses (SOG). The photoresists and polyimides remain as hardened organic materials after curing. The SOGs consist of alcohol soluble silicates and siloxanes which can attain the properties of inorganic glasses when properly cured. The SOGs have found wide acceptance in recent sub-micron semiconductor processing technology because of their low defect density, simplified processing, and low thermal budget. However, the SOG layers are limited to thicknesses of only a few thousand Angstroms. Thicker layers tend to crack and require longer curing times.
A common method of usage is to first deposit a layer of PECVD silicon oxide over the layer of patterned metallization and then lay the SOG over it. The SOG fills in the narrow features. It is then etched back by anisotropic etching to the PECVD oxide surface. The result is the original PECVD layer with the small spacings filled with SOG.
The SOG is deposited by a nozzle directed at the center of a rapidly spinning wafer. Centrifugal force distributes the liquid over the wafer. Excess liquid is flung from the edge of the wafer. The apparatus used to perform this task is of the same type that is conventionally used to deposit photoresist. The wafer is then allowed to dry. This method of application results in a layer that is somewhat thicker at the outer edge of the wafer. During the subsequent RIE etch back step, a circular clamp is used hold the wafer in place. This clamp shields a portion of the SOG preventing its removal during the etch back procedure. If this material is not removed prior to the etch back, it can crack during or after the SOG etch back thereby releasing particulate contamination.
In order to remove this edge band of SOG, an edge rinse with isopropyl alcohol(IPA) is performed prior to curing. The wafer is spun on the apparatus used to apply the SOG. The IPA is applied by a nozzle directed at the wafer edge. IPA effectively removes sufficient SOG from the edge region but in doing so tends to attack the SOG on portions of usable dice lying in close proximity to the wafer edge. These dice, which lie within the manufacturing process edge tolerance but are partly exposed to the IPA edge rinse would normally be lost.